The invention relates to a device for automatic detection of a possible incorrect measurement. In particular, the invention relates to the automatic detection of a possible incorrect measurement in a coordinate measuring machine, wherein the device comprises at least one reflected light illumination apparatus and/or a transmitted light illumination apparatus and at least one imaging optical system and one detector of a camera for imaging structures on a mask (substrate), wherein a first program portion is linked to the detector of the camera, said detector being provided for determining the position and/or dimension of the structure on the mask, wherein the device determines and records a plurality of measurement variables Mj, jε{1, . . . , L}, from which at least one variable G can be determined. G can be the position or the structural width (CD, critical dimension) of the structure on the mask.
The invention also relates to a method for automatic detection of a possible incorrect measurement. In particular, the invention relates to a method for automatic detection of a possible incorrect measurement in a coordinate measuring machine, wherein at least one structure on the mask for semiconductor production is illuminated with at least one reflected light illumination apparatus and/or one transmitted light illumination apparatus, the structure on the mask is imaged by at least one imaging optical system on a detector of a camera, during measurement of the structure on the mask in relation to the position and/or dimension the values are determined by means of a first program portion that is linked to the detector of the camera, and wherein a plurality of measurement variables Mj, jε{1, . . . , L}, is determined and recorded, wherefrom at least one variable G is determined.
It is fundamentally impossible to make measurements without errors. Due to various causes, the variable G to be measured is not correctly detected. An incorrect measurement (measurement deviation) is understood to mean a measurement value that has a greater deviation from the true value of the variable G to be measured than is the case according to the statistical average. A position measurement can be disrupted, for example, by the sudden opening of a door or by floor vibration and thereby lead to incorrect measurements. A number of other parameters influence the measured position or CD of a structure on the substrate. These parameters may be, for example, pressure, temperature, the tilt angle or profile shape of the structure on the substrate, deviations in the intensity of the measured profile, etc. Apart from the microscope with a camera as described below, at least one laser distance measuring system (X-Y etalon axis) is needed.
The measurement of structures on a substrate, such as a mask, is carried out with a coordinate measuring machine. A coordinate measuring machine of this type is sufficiently well known from the prior art. Reference is made in this regard, for example, to the lecture manuscript “Pattern Placement Metrology for Mask Making” by Dr. Carola Bläsing. The lecture was given at the Semicon Education Programme congress in Geneva on 31 Mar. 1998, and describes the coordinate measuring machine in detail.
Since the present invention can be advantageously used in a measuring device of this type, the embodiments of the present invention on the following pages are described—without restricting its scope—primarily in relation to a measuring device of this type. In the context of the present invention, the terms “sample”, “substrate”, “mask” and the general expression “object” are taken to have equivalent meanings. The coordinate measuring machine is placed in a climate chamber in order to be able to achieve measuring accuracy in the nanometer range. What is measured is the coordinates of structures on masks and wafers. The measuring system is arranged on a block mounted in vibration damping manner. The block is preferably constructed as a granite block. The masks and wafers are mounted on a measuring table with an automatic handling system. In the production of semiconductor chips arranged on wafers, with increasing component density, the width of the individual structures becomes ever smaller. Therefore the demands placed on the specifications of coordinate measuring machines used as measuring and inspection systems for measuring the edges and the positions of the structures and for measuring the CD also increase. As previously, optical sensing methods in conjunction with laser displacement measuring systems are still favoured for these machines although the required measuring accuracy (currently in the range of a few nanometers) lies far below the resolving power achievable with the light wavelength used (light with a wavelength of less than or equal to 400 nm). The advantage of optical measuring equipment lies in the substantially less complex construction as well as its simplicity in use compared with systems using other sensing methods or with X-ray or electron beams.
The construction of a coordinate measuring machine of this type, as known, for example, from the prior art will now be described in greater detail by reference to FIG. 1. A method and a measuring device for determination of the position of structures on a substrate are known from the unexamined German published application DE 10047211 A1. Therefore with regard to details of the position determination as described, reference should be expressly made to this document. However, the coordinate measuring machine is not suitable for calculating an analysis of specified measurement variables Mj with regard to a possible incorrect measurement.
The German patent application DE 19819492 describes a measuring machine for structural widths or the position of structures on the substrate. Herein, the measuring table slides on air bearings on the surface of the granite block. Mounted on two mutually perpendicular sides of the measuring table are planar mirrors. A laser interferometer system determines the position of the measuring table. Some other clean room-compatible guidance of the measuring table is also conceivable. The illumination and the imaging of the structures to be measured is carried out with a high-resolution apochromatically corrected microscope lens and with reflected light or transmitted light in the spectral range of the near UV or light having a wavelength of less than or equal to 400 nm. CCD camera serves as the detector. Measurement signals are obtained from the pixels of the detector array that lie within a measuring window. From this, an intensity profile of the measured structure is derived by means of image processing, from which, for example, the edge position of the structure is determined.
The measured edge position depends on the physical quality of the edge itself, on the optical measuring method used, and also on the quality of the imaging system. The correlation is described in the document “Edge Measurement on Microstructures”, by W. Mirandé, VDI Reports No. 1102 (1993), pages 137 ff. If the position of the structure is defined by the midline between the two edges, in general, the factors influencing the measured edge position have no effect on the measured position of the structure. However, evaluation of the measurement results for a measurement of structure width can lead to different results in different measuring devices.
During semiconductor production, the mask is illuminated in the stepper using transmitted light illumination and imaged on the wafer. It is therefore of interest to be able to determine the precise light-shading width of the structural element. Special measuring microscopes with which the mask is illuminated using transmitted light and only the width of the opaque structure is measured have been developed for this purpose. These measuring devices are not provided for determining the position coordinates of the structural elements. These considerations apply in the same way if, in place of opaque structural elements, transparent structural elements in the mask surface are to be measured.
The German patent application DE 10 2005 009 536 A1 describes a method for mask inspection which can be used within the context of mask design during mask production, in order to identify relevant weak-spots and correct them at an early stage. It is thus intended that errors should be identified in the mask layout and the mask design so that the masks that are produced have a smaller error quota and therefore costs are minimised.
The AIMS<TM> (Aerial Imaging Measurement System) from Carl Zeiss SMS GmbH has been established on the market for 10 years for the analysis of mask defects with regard to printability. Herein a small region of the mask (the defect site with its surroundings) is illuminated and imaged using the same illumination and imaging conditions (wavelength, NA (numerical aperture), illumination type, coherence level of the light (Sigma)) as in a lithographic scanner. In contrast to a scanner, however, the aerial image that is formed of the mask is enlarged onto a CCD camera. The camera sees the same latent image as the photoresist on the wafer. Therefore the aerial image can be analysed without complex test prints using wafer exposure devices and conclusions can be drawn about the printability of the mask. By recording a focus series, additional information is gathered regarding the analysis of the lithographic processing window (in this regard, see DE10332059 A1).
The German patent application DE 10332059 A1 describes a method for analysing objects in microlithography, preferably on masks, by means of an Aerial Image Measurement System (AIMS), which comprises at least two imaging stages, wherein the detected image is corrected with regard to transmission behaviour by means of a correction filter of the second or further imaging stages and the illumination of the object is carried out using reflected and/or transmitted light illumination, wherein the correction is carried out in a manner such that the corrected starting variables correspond to those of an image from a photolithography stepper or scanner, wherein the correction takes place by folding back and measured or calculated correction values are used for the correction.
The European patent application EP 0 628 806 describes a device and a method for determining the characteristics of a photolithographic mask. In the AIMS mask inspection microscope, for example, the setting and observation of certain illumination settings is linked thereto. The illumination light in this case comes from the UV region. The detector or the image recording unit is a UV CCD camera.
The international patent application WO 00/60415 A1 describes a method for correcting imaging errors, wherein by modifying an electronic mask design, following illumination of this mask design, structures that approach the original mask design or the intended mask as closely as possible are imaged on the mask with a mask writer. The process conditions that need to be taken into account are summarised in the form of tables containing, in particular, the parameters which depend on the process conditions, given in the form of correction values. For example, the tables include site-dependent correction values for compensation of the device-specific aberration of the mask writer. The solution is based on physically founded models of the respective imaging errors. With the proposed method, in contrast to known methods, it is possible to correct effectively mask structures designed for generating highly integrated circuits, although the calculational effort involved is significant. A further disadvantage of this method lies in the large number of parameters that must be taken into account in the form of correction values. Furthermore, in addition to diffraction and refraction effects, interactions and device-specific aberration effects must also be taken into account.
The unpublished German patent application DE 10 2007 028 260 describes a device for measuring positioning and structural widths or at least one structure on a surface of a substrate. The substrate is set into the measuring table in such a manner that a normal vector pointing out of the surface of the substrate which bears the structures is essentially parallel to the gravitational force vector.
None of the devices and/or methods known from the prior art is able to identify incorrect measurements so reliably that the throughput of the coordinate measuring machine is only insubstantially reduced, and to a degree that is acceptable to the user.